## 20.6. Writing Command Lines

Writing good, portable `Makefile` files is a bit of an art. Skill comes with practice and experience. Here are some tips to get you started:

- Naming your file `Makefile` instead of `makefile` usually causes it to be listed first with `ls`. This makes it easier to find in a directory with many files.

- Remember that command lines must start with a leading tab character. You cannot just indent the line with spaces, even eight spaces. If you use spaces, `make` exits with an unhelpful message about “missing separator characters.”

- Remember that `$` is special to `make`. To get a literal `$` into your command lines, use `$$`. This is particularly important if you want to access an environment variable that isn't a `make` macro. Also, if you wish to use the shell's `$$` for the current process ID, you have to type it as `$$$$`.

- Write multiline shell statements, such as shell conditionals and loops, with trailing semicolons and a trailing backslash:

  >     if [ -f specfile ] ; then \
  >     ... ; \
  >     else \
  >     ... ; \
  >     fi

  Note that the shell keywords `then` and `else` don't need the semicolon. (What happens is that `make` passes the backslashes and the newlines to the shell. The escaped newlines are not syntactically important, so the semicolons are needed to separate the different parts of the command. This can be confusing. If you use a semicolon where you would normally put a newline in a shell script, things should work correctly.)

- Remember that each line is run in a separate shell. This means that commands that change the shell's environment (such as `cd`) are ineffective across multiple lines. The correct way to write such commands is to separate commands on the same line with a semicolon:

  >     cd subdir; $(MAKE)

- For guaranteed portability, always set `SHELL` to `/bin/sh`. Some versions of `make` use whatever value is in the environment for `SHELL`, unless it is explicitly set in the `Makefile`.

- Use macros for standard commands. `make` already helps out with this, providing macros such as `$(CC)`, `$(YACC)`, and so on.

- When removing files, start your command line with `-$(RM)` instead of `$($RM)`. (The – causes `make` to ignore the exit status of the command.) This way, if the file you were trying to remove doesn't exist, and `rm` exits with an error, `make` can keep going.

- When running subsidiary invocations of `make`, typically in subdirectories of your main program tree, always use `$(MAKE)`, and not `make`. Lines that contain `$(MAKE)` are always executed, even if `-n` has been provided, allowing you to test out a whole hierarchy of `Makefile` files. This does not happen for lines that invoke `make` directly.

- Often, it is convenient to organize a large software project into subprojects, with each one having a subdirectory. The top-level `Makefile` then just invokes `make` in each subdirectory. Here's the way to do it:

  >     SUBDIRS = proj1 proj2 proj3
  >     ...
  >     projects: $(SUBDIRS)
  >     	for i in $(SUBDIRS); \
  >     	do \
  >     		echo ====== Making in $$i ; \
  >     		( cd $$i ; $(MAKE) $(MAKEFLAGS) $@ ) ; \
  >     	done

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